1. Field of the Invention
The present invention relates to an image processing device and an image processing method which are used for an ink jet recording apparatus and an ink jet recording apparatus comprising the image processing device.
2. Description of the Related Art
In recent years, an ink jet recording apparatus which jets ink from each nozzle of a recording head having a plurality of nozzles that jets the ink to a recording medium to record a desired image on the recording medium while moving the recording head with respect to the recording medium has come into widespread use.
A serial ink jet recording apparatus has been generally known as the ink jet recording apparatus. In the serial ink jet recording apparatus, a recording head jets ink to record an image in a region with a predetermined length in a sub-scanning direction while being moved in a main scanning direction. Then, a recording medium is moved by a predetermined distance in the sub-scanning direction and the recording head records an image in the next region in the same manner. Then, this process is repeatedly performed. In this way, images are recorded on the entire region of the recording medium.
The serial ink jet recording apparatus records images while repeating the movement (which is also referred to as printing pass) of the recording head in the main scanning direction a plurality of times. Therefore, the behavior of dots on the recording medium varies depending on errors in the recording position of each dot on the recording medium from each nozzle of the recording head, errors in the amount of ink jetted, or the recording order and time of dots. As a result, in some cases, density unevenness that is called “banding” in which density varies with the repetition period of each printing pass or the boundary between the printing passes is conspicuous occurs. When banding occurs, print quality deteriorates.
JP2002-096455A discloses an ink jet recording apparatus which thins out image data subjected to halftone processing, using a mask pattern corresponding to each nozzle, to thin out the nozzles that jet ink, when the ink is jetted from each nozzle of a recording head. In the ink jet recording apparatus, the mask pattern is set such that the proportion (that is, a jetting rate) of pixels recorded by the nozzles close to the end of the nozzle array of the recording head is less than that of the pixels recorded by the nozzles close to the center of the nozzle array. In this way, banding is prevented.
JP2010-162770A discloses an ink jet recording apparatus which performs halftone processing with an ordered dither method using a dither mask to control the jetting of ink from each nozzle of a recording head. In the ink jet recording apparatus, the threshold value of the dither mask is set such that the jetting rate of at least one of nozzle groups (including the outermost nozzles disposed at both ends) which are disposed at both ends of a nozzle column of a recording head is less than the jetting rate of an intermediate nozzle group which is disposed between the nozzle groups at both ends. In this way, banding is prevented.